The present invention relates to an electromagnetic shielding member for preventing electromagnetic wave leakage and entrance and, more particularly, to an electromagnetic shielding member which is used for a seam portion of a metal case or a door portion of an electromagnetic shielding chamber so as to suppress electromagnetic waves leaking from a contact portion.
When electromagnetic shielding is to be performed, since, in general, the conductivity of a portion at which conductive members such as metal members are in contact with each other is partially decreased, an electromagnetic shielding member must be arranged at the contact portion between the conductive members in order to suppress leaking electromagnetic waves.
As conventional electromagnetic shielding members of this type, for example, the following members have been used: a spring finger gasket 31 constituted by a comb-like metal piece shown in a cross-sectional view of FIG. 20A and in a perspective view of FIG. 20B; and an EMI gasket 32 constituted by a conductive rubber tube shown in a cross-sectional view of FIG. 21A and in a perspective view of FIG. 21B ("Summary of Electromagnetic Shield and Earth System", Nihon Kogyo Gijutsu Center). The electromagnetic shielding member shown in FIGS. 20A and 20B is inserted between contact portions of conductive members by using the biasing force of the metal piece, whereas the electromagnetic shielding member shown in FIGS. 21A and 21B is inserted between the conductive portions by using the elasticity of the conductive rubber, thus increasing conductivity.
In a microwave oven, a ferrite member 33 is arranged in a gap between a main body 34 and a door 35 as shown in FIG. 22, in order to absorb leaking electromagnetic waves ("THE SHIELDING EFFECT OF WAVE ABSORBER ON LEAKAGE THROUGH THE GAP OF MICROWAVE OVEN" by Yasutaka Shimizu, 1984 INTERNATIONAL SYMPOSIUM ON ELECTROMAGNETIC COMPATIBILITY, IECE, pps. 540-544).
The conductive electromagnetic shielding member shown in FIGS. 20A and 20B or FIGS. 21a and 21B is designed such that the spring finger gasket 31 or the gasket 32 must be brought into direct contact with surfaces of conductive members. Therefore, if coatings are to be applied to the conductive members, non-coated portions are required on the surfaces of the conductive members because the coatings may serve as insulating members. If the spring finger gasket 31 and the gasket 32 are not used, since a contact state between a case and a mounting portion of an electromagnetic shielding chamber influences electromagnetic wave leakage, a mechanism for reinforcing the contact state is required. In addition, if the spring finger gasket 31 and gasket 32 are used, degradation in shielding effectiveness is caused by changes in biasing force and elasticity with a lapse of time. Furthermore, as indicated by a curve a in FIG. 14, a shielding effectiveness is reduced in a low-frequency band in which electromagnetic wave source has a relatively low impedance.
In the method of inserting the electromagnetic wave absorbing member, as FIG. 9A shows its structure and a curve a in FIG. 8 indicates a frequency characteristic of a shielding effectiveness corresponding to the structure, only a small amount of electromagnetic waves is absorbed and hence a satisfactory leakage suppressing effect cannot be obtained in a low-frequency band in which the length of a gap in the popagation direction becomes very small in comparison with a wavelength.